Inkjet printing systems are generally of two types: drop-on-demand (DOD) printing systems and continuous inkjet (CIJ) printing systems. Inkjet printing is a standard method for printing an image such as a colored image onto a substrate wherein a stream of ink droplets is directed from a printing device to a receptive surface of a suitable receiver element or substrate. The direction of the stream of droplets can be controlled electronically causing the droplets to print the desired image or information on the substrate without requiring contract between the printing device and the receptive surface to which the ink is applied. Objects comprising substrates to which inkjet printing is well suited include but are not limited to, containers for consumer products, currency, draft checks, envelopes, letterhead, identification cards, lottery tickets, bank cards, identification strips, labels, brochures, signage, and other well-known materials.
Drop-on-demand printing systems are widely used in home or consumer inkjet printers and slower consumer printers, both of which have been available for several decades. As the name implies, this type of inkjet printing uses a print head that ejects drops of ink only when signaled to do so by a digital controller.
CIJ printing systems generally comprise two main components, a fluid system (including a recirculating main fluid supply or ink reservoir) and one or more print heads. Ink can be pumped through a supply line from the recirculating main fluid supply to a manifold that distributes the ink to a plurality of orifices, typically arranged in linear array(s), under sufficient pressure to cause ink streams to issue from the orifices of the print head(s). Stimulations can be applied to the print head(s) to cause those ink streams to form streams of uniformly sized and spaced drops that are deflected in a suitable manner, into printing or non-printing paths. Non-printing drops can be returned to the recirculating main fluid supply using a drop catcher and a return line. In contrast to DOD printing systems, CIJ printing systems can involve the use of a continuous stream of ink drops that are separated to discriminate between spaced printing drops and non-printing drops. This discrimination can be accomplished by electrostatically charging the drops and passing the charged drops through an electric field. Charged drops are deflected by a charge field and can be caught and returned to the reservoir of ink. Uncharged drops are printed on a substrate or receiver material. This discrimination can also be accomplished when the printhead digitally creates large and small drops. Small drops are deflected by an air current and returned to the reservoir of ink. Large drops, being less affected by the air current, are printed onto a substrate or receiver material. Some useful CIJ printing apparatus and printhead fabrication are described for example in U.S. Pat. No. 6,588,888 (Jeanmaire et al.) and U.S. Pat. No. 6,943,037 (Anagnostopoulos et al.).
Commercially available CIJ inks are mostly aqueous dye-based inks that exhibit a number of problems. In such dye-based inks, no particles are observable under the microscope. Although there have been many recent advances in the art of dye-based inkjet inks, such inks still suffer from deficiencies such as low optical densities on coated glossy paper and poor light-fastness. When water is used as the carrier, such inks also generally suffer from poor water fastness, poor smear resistance, and uncontrolled gloss.
To address many of these problems, pigment based inks have been developed and incorporated into commercial CIJ systems as noted in the patents cited above and the various publications cited therein. In addition, U.S. Pat. No. 7,537,650 (Szajewski et al.) describes inkjet sets containing a colored aqueous ink (with cationic coloring agent or pigment) and a substantially colorless aqueous ink that contains an anionic polymer or oligomer. Such inks can be applied in substantially an overlaying manner. Both inks are generally applied to the same areas of a receiver element so that the mixing of cationic and anionic materials in the inks provide a more consistent single color density on a variety of substrates, thereby reducing intercolor bleed and non-uniformities.
In general, such pigment-based colored inks can comprise a wide variety of colored pigments that can be chosen depending upon the specific application and performance requirements for the printing system and desired printing results. For example, such pigments can include but are not limited to, carbon black or other black pigments, red pigments, green pigments, blue pigments, orange pigments, violet pigments, magenta pigments, yellow pigments, and cyan pigments. The printed images using such pigment-based inks are generally desired to have a visual density of at least 0.5.
Colorless or invisible aqueous inkjet printer ink compositions containing various fluorescing pigments, and optionally colored non-fluorescing pigments, are described in U.S. Pat. No. 8,349,211 (Cai et al.) and in U.S. Patent Application Publication 2014/0231674 (Cook).
Various additives have been incorporated into both colored and colorless inkjet ink compositions to provide desired physical and chemical properties. For example, in some instances a defoamer (or antifoamant or defoaming agent) can be included in small amounts to reduce propensity to foaming during manufacture or use. It has been observed that when aqueous inkjet ink compositions are supplied from a recirculating main fluid supply and perhaps passed through one or more filters, known defoamers having a hydrophilic-lipophilic balance value below 3, are insufficient to keep foaming under control and as a result, such aqueous inkjet ink compositions cannot be readily used in such recirculating systems. Foaming can cause overflow of recirculating main fluid supplies and significant waste. It would be desirable to solve this problem so that foaming is not a problem in such recirculating ink jetting systems.